• A first-type limit line exists on the worm wheel tooth surface and is normally located out of the meshing zone, indicating the undercutting boundary.
• The undercutting mechanism occurs when part of the meshing zone near the conjugated line of the worm tooth crest enters the undercutting area and is cut off during machining.
• Only one intersection point exists between the first- and second-type limit lines, which is identified as a lubrication weak point.
• A convenient characteristic quantity based on first-type limit function values on the worm tooth crest is proposed to judge whether undercutting exists across the whole meshing zone.